This seems obvious in retrospect, but I got this from Olivier Wietrich's tag-reduce.

Tagged Literal Templates are useful syntactic sugar. I fill out a lot of school physical forms (each school has its own form), so I need strings like `Weight: ${data.wt_lb} pounds (${data.wt_kg} kg), ${data.wt_pct} percentile`. That works fine if I am evaluating the template string when it is defined.

But I want to defer evaluating those strings until the data array is filled in (by parsing my EMR's note, a topic for another time).

Mozilla suggests something like

function template(strings, ...keys) {
  return (dict) => {
    const result = [strings[0]];
    keys.forEach((key, i) => {
      result.push(dict[key], strings[i + 1]);
    return result.join('');

and using as

weightTemplate = template`Weight: ${'wt_lb'} pounds (${'wt_kg'} kg), ${'wt_pct'} percentile`;
// later in code, when data dictionary is completed
finalString = weightTemplate (data);

But Olivier Wietrich suggests the much more elegant:

function template(strings, ...keys) {
  return (data) => strings.reduce ( (previousValue, currentValue, currentIndex) => previousValue + data[keys[currentIndex-1]] + currentValue);
  // (I'm using ECMA's parameter names for reduce, which are pretty verbose)

using Array.prototype.reduce, which should have been obvious.
Using template is the same as above.

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